Contents of the FAQ-C.TXT file

A: For each pointer type, there is a special value -- the "null pointer" -- which is distinguishable from all other pointer values and which is not the address of any object.

2. How do I "get" a null pointer in my programs?

A: A constant 0 in a pointer context is converted into a null pointer at compile time. A "pointer context" is an initialization, assignment, or comparison with one side a variable or expression of pointer type, and (in ANSI standard C) a function argument which has a prototype in scope declaring a certain parameter as being of pointer type. In other contexts (function arguments without prototypes, or in the variable part of variadic function calls) a constant 0 with an appropriate explicit cast is required.

3. What is NULL and how is it #defined?

A: NULL is simply a preprocessor macro, #defined as 0 (or (void *)0), which is used (as a stylistic convention, in favor of unadorned 0's) to generate null pointers,

4. How should NULL be #defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?

A: The same as any other machine: as 0 (or (void *)0). (The compiler makes the translation, upon seeing a 0, not the preprocessor.)

5. If NULL were defined as "(char *)0," wouldn't that make function calls which pass an uncast NULL work?

A: Not in general. The problem is that there are machines which use different internal representations for pointers to different types of data. A cast is still required to tell the compiler which kind of null pointer is required, since it may be different from (char *)0.

6. I use the preprocessor macro "#define Nullptr(type) (type *)0" to help me build null pointers of the correct type.

A: This trick, though valid, does not buy much.

7. Is the abbreviated pointer comparison "if(p)" to test for non-null pointers valid? What if the internal representation for null pointers is nonzero?

A: The construction "if(p)" works, regardless of the internal representation of null pointers, because the compiler essentially rewrites it as "if(p != 0)" and goes on to convert 0 into the correct null pointer.

8. If "NULL" and "0" are equivalent, which should I use?

A: Either; the distinction is entirely stylistic.

9. But wouldn't it be better to use NULL (rather than 0) in case the value of NULL changes, perhaps on a machine with nonzero null pointers?

A: No. NULL is, and will always be, 0.

10. I'm confused. NULL is guaranteed to be 0, but the null pointer is not?

A: A "null pointer" is a language concept whose particular internal value does not matter. A null pointer is requested in source code with the character "0". "NULL" is a preprocessor macro, which is always #defined as 0 (or (void *)0).

11. Why is there so much confusion surrounding null pointers? Why do these questions come up so often?

A: The fact that null pointers are represented both in source code, and internally to most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL) suggests that the value might change later, or on some weird machine.

12. I'm still confused. I just can't understand all this null pointer stuff.

A: A simple rule is, "Always use `0' or `NULL' for null pointers, and always cast them when they are used as arguments in function calls."

13. Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to be represented internally by zeroes?

A: Since functions can never receive arrays as parameters, any parameter declarations which "look like" arrays are treated by the compiler as if they were pointers.

20. Someone explained to me that arrays were really just constant pointers.

A: An array name is "constant" in that it cannot be assigned to, but an array is _not_ a pointer.

21. I came across some "joke" code containing the "expression" 5["abcdef"] . How can this be legal C?

A: Yes, array subscripting is commutative in C. The array subscripting operation a[e] is defined as being equivalent to *((a)+(e)).

22. My compiler complained when I passed a two-dimensional array to a routine expecting a pointer to a pointer.

A: The rule by which arrays decay into pointers is not applied recursively. An array of arrays (i.e. a two-dimensional array in C) decays into a pointer to an array, not a pointer to a pointer.

23. How do I declare a pointer to an array?

A: Usually, you don't want to. Consider using a pointer to one of the array's elements instead.

24. How can I dynamically allocate a multidimensional array?

A: It is usually best to allocate an array of pointers, and then initialize each pointer to a dynamically-allocated "row." See the full list for code samples.

Section 3. Order of Evaluation

25. Under my compiler, the code "int i = 7; printf("%d\n", i++ * i++);" prints 49. Regardless of the order of evaluation, shouldn't it print 56?

A: The operations implied by the postincrement and postdecrement operators ++ and -- are performed at some time after the operand's former values are yielded and before the end of the expression, but not necessarily immediately after, or before other parts of the expression are evaluated.

26. But what about the &&, ||, and comma operators?

A: There is a special exception for those operators, (as well as ?: ); left-to-right evaluation is guaranteed.

Section 4. ANSI C

27. What is the "ANSI C Standard?"

A: In 1983, the American National Standards Institute commissioned a committee, X3J11, to standardize the C language. After a long, arduous process, the committee's work was finally ratified as an American National Standard, X3.159-1989, on December 14, 1989, and published in the spring of 1990. The Standard has also been adopted as ISO/IEC 9899:1990.

28. How can I get a copy of the Standard?

A: Copies are available from the American National Standards Institute in New York, or from Global Engineering Documents in Irvine, CA. See the unabridged list for addresses.

29. Does anyone have a tool for converting old-style C programs to ANSI C, or for automatically generating prototypes?

A: The former is a pointer to a constant character; the latter is a constant pointer to a character.

31. My ANSI compiler complains about a mismatch when it sees

extern int func(float);

int func(x) float x; {...

A: You have mixed the new-style prototype declaration "extern int func(float);" with the old-style definition "int func(x) float x;". The problem can be fixed by using either new-style (prototype) or old-style syntax consistently.

A: Under ANSI C, #ifdeffed-out text must still consist of "valid preprocessing tokens." This means that there must be no unterminated comments or quotes (i.e. no single apostrophes), and no newlines inside quotes.

33. Why does the ANSI Standard not guarantee more than six monocase characters of external identifier significance?

A: The problem is older linkers which cannot be forced (by mere words in a Standard) to upgrade.

34. Whatever happened to noalias?

A: It was deleted from the final versions of the standard because of widespread complaint and the near-impossibility of defining it properly.

35. What are #pragmas and what are they good for?

A: The #pragma directive provides a single, well-defined "escape hatch" which can be used for extensions.

Section 5. C Preprocessor

36. How can I write a generic macro to swap two values?

A: There is no good answer to this question. The best all-around solution is probably to forget about using a macro.

37. I have some old code that tries to construct identifiers with a macro like "#define Paste(a, b) a/**/b", but it doesn't work any more.

A: alloca allocates memory which is automatically freed when the function which called alloca returns. alloca cannot be written portably, is difficult to implement on machines without a stack, and fails under certain conditions if implemented simply.

Section 9. Structures

54. I heard that structures could be assigned to variables and passed to and from functions, but K&R I says not.

A: These operations are supported by all modern compilers.

55. How does struct passing and returning work?

A: If you really need to know, see the unabridged list.

56. I have a program which works correctly, but dumps core after it finishes. Why?

A: Check to see if a structure type declaration just before main is missing its trailing semicolon, causing the compiler to believe that main returns a struct. See also question 96.

57. Why can't you compare structs?

A: There is no reasonable way for a compiler to implement struct comparison which is consistent with C's low-level flavor.

58. I came across some code that declared a structure with the last member an array of one element, and then did some tricky allocation to make the array act like it had several elements. Is this legal and/or portable?

A: The ANSI C standard allows it, but only implicitly.

59. How can I determine the byte offset of a field within a structure?

A: ANSI C defines the offsetof macro, which should be used if available.

60. How can I access structure fields by name at run time?

A: Build a table of names and offsets, using the offsetof() macro.

Section 10. Declarations

61. How do you decide which integer type to use?

A: If you might need large values, use long. If space is very important, use short. Otherwise, use int.

62. I can't seem to define a linked list node which contains a pointer to itself.

A: Structs in C can certainly contain pointers to themselves; the discussion and example in section 6.5 of K&R make this clear. Problems arise if an attempt is made to define (and use) a typedef in the midst of such a declaration; avoid this.

63. How do I declare an array of pointers to functions returning pointers to functions returning pointers to characters?

A: char *(*(*a[5])())(); Using a chain of typedefs, or the cdecl program, makes these declarations easier.

64. So where can I get cdecl?

A: Several public-domain versions are available. See the full list for details.

65. How do I initialize a pointer to a function?

A: Use something like "extern int func(); int (*fp)() = func; " .

66. I've seen different methods used for calling through pointers to functions.

A: The extra parentheses and explicit * are now officially optional, although some older implementations require them.

Section 11. Boolean Expressions and Variables

67. What is the right type to use for boolean values in C? Why isn't it a standard type? Should #defines or enums be used for the true and false values?

A: C does not provide a standard boolean type, because picking one involves a space/time tradeoff which is best decided by the programmer. The choice between #defines and enums is arbitrary and not terribly interesting.

68. Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered "true" in C? What if a built-in boolean or relational operator "returns" something other than 1?

A: It is true (sic) that any nonzero value is considered true in C, but this applies only "on input", i.e. where a boolean value is expected. When a boolean value is generated by a built-in operator, it is guaranteed to be 1 or 0. (This is _not_ true for some library routines such as isalpha.)

69. What is the difference between an enum and a series of preprocessor #defines?

A: At the present time, there is little difference. The ANSI standard states that enumerations are compatible with integral types.

Section 12. Operating System Dependencies

70. How can I read a single character from the keyboard without waiting for a newline?

A: Contrary to popular belief and many people's wishes, this is not a C-related question. How to do so is a function of the operating system in use.

71. How can I find out if there are characters available for reading (and if so, how many)? Alternatively, how can I do a read that will not block if there are no characters available?

A: These, too, are entirely operating-system-specific.

72. How can my program discover the complete pathname to the executable file from which it was invoked?

A: argv[0] may contain all or part of the pathname. You may be able to duplicate the command language interpreter's search path logic to locate the executable.

73. How can a process change an environment variable in its caller?

A: In general, it cannot.

74. How can a file be shortened in-place without completely clearing or rewriting it?

A: BSD systems provide ftruncate(), and several others supply chsize(), but there is no truly portable solution.

Section 13. Stdio

75. Why does errno contain ENOTTY after a call to printf?

A: Don't worry about it. It is only meaningful for a program to inspect the contents of errno after an error has occurred.

76. My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program.

A: It is best to use an explicit fflush(stdout) whenever output should definitely be visible.

77. When I read from the keyboard with scanf(), it seems to hang until I type one extra line of input.

A: scanf() was designed for free-format input, which is seldom what you want when reading from the keyboard.

78. How can I recover the file name given an open file descriptor?

A: This problem is, in general, insoluble. It is best to remember the names of open files yourself.

Section 14. Style

79. Is the code "if(!strcmp(s1, s2))" good style?

A: No.

80. What's the best style for code layout in C?

A: There is no one "best style," but see the full list for a few suggestions.

81. Where can I get the "Indian Hill Style Guide" and other coding standards?

A: See the unabridged list.

Section 15. Miscellaneous

82. What can I safely assume about the initial values of variables which are not explicitly initialized?

A: Variables with "static" duration start out as 0, as if the programmer had initialized them. Variables with "automatic" duration, and dynamically-allocated memory, start out containing garbage (with the exception of calloc).

83. Can someone tell me how to write itoa?

A: Just use sprintf.

84. How can I convert a struct tm or a string into a time_t?

A: The ANSI mktime routine converts a struct tm to a time_t. No standard routine exists to parse strings.

85. How can I write data files which can be read on other machines with different data formats?

A: The best solution is to use text files.

86. I seem to be missing the system header file . Can someone send me a copy?

A: You cannot just pick up a copy of someone else's header file and expect it to work, since the definitions within header files are frequently system-dependent. Contact your vendor.

87. How can I call Fortran (BASIC, Pascal, ADA, lisp) functions from C?

A: The answer is entirely dependent on the machine and the specific calling sequences of the various compilers in use.

88. Does anyone know of a program for converting Pascal (Fortran, lisp, "Old" C, ...) to C?

A: Several public-domain programs are available, namely ptoc, p2c, and f2c. See the full list for details.

89. Where can I get copies of all these public-domain programs?

A: See the regular postings in the comp.sources.unix and comp.sources.misc newsgroups for information.

90. When will the next Obfuscated C Contest be held? How can I get a copy of the previous winning entries?

A: Precise answers to these and many similar questions depend of course on the processor and compiler in use.

94. My floating-point calculations are acting strangely and giving me different answers on different machines.

A: See the full list for a brief explanation, or any good programming book for a better one.

95. I'm having trouble with a Turbo C program which crashes and says something like "floating point not loaded."

A: Some compilers for small machines, including Turbo C, attempt to leave out floating point support if it looks like it will not be needed. The programmer must occasionally insert a dummy explicit floating-point call to force loading of floating-point support.

96. This program crashes before it even runs!

A: Look for very large, local arrays. (See also question 56.)

97. Does anyone have a C compiler test suite I can use?

A: Plum Hall, among others, sells one.

98. Where can I get a YACC grammar for C?

A: See the ANSI Standard, or the unabridged list.

99. How do you pronounce "char"?

A: Like the English words "char," "care," or "car" (your choice).

100. Where can I get extra copies of this list?

A: For now, just pull it off the net; the unabridged version is normally posted on the first of each month, with an Expiration: line which should keep it around all month.